Martensitic steel



United States Patent 3,288,611 MARTENSITIC STEEL Remus A. Lula, Natrona Heights, and Harry E. McCune III, Brackenridge, Pa., assignors to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Filed Oct. 14, 1963, Ser. No. 316,116

4 Claims. (Cl. 75-128) This invention relates to an improved high strength martensitic stainless steel and in particular to a m-arten sitic high strength stainless steel having an optimum combination of form'ability, heat treatment, welding and corrosion resistance characteristics.

Within the last decade stainless steels have assumed an increasingly important role as structural materials, especially in the transportation industry. These stainless steels, well known for some time, have recently been applied in this field because of their combination of strength and corrosion resistance. Of the stainless steels available, the A181 200, 300 and 400 Series have been utilized, depending upon the combination of properties desired. For example, where high strength with minimum corrosion resistance was desirable, the A151 400 Series martensitic stainless steels have been utilized, an example of which is AISI Type 410. The use of the martensitic stainless steels has been fairly limited, however, because of the difiiculties encountered from the fabrication standpoint. In this respect the martensitic stainless steels were characterized by having a limited amount of cold workability, necessitating rather long heat treatments at subcritical temperatures in order to confer an adequate deg-rec of cold workability to the steel. Moreover, when fusion welding was employed in fabricating these steels into structural components both p-rewelding and postwelding heat treatments were necessary in order to confer upon the steel an adequate degree of strength and ductility in actual use. These steels are characterized by having a relatively high carbon content for strength purposes and a low chromium content in order to avoid the formation of delta ferrite therein. As a result, the corrosion resistance of these materials was severely limited.

On the other hand, where high corrosion resistance was desired in a stainless steel for use as a structural component, it was necessary to rely upon the austenitic-type stainless steels, an example of which is AISI Type 302, or the ferritic stainless steels, an example of which is AISI Type 430. These steels are characterized by having good formability, corrosion resistance and welding properties; however, their strength is limited since they cannot be hardened by heat treatment alone. In order to obtain adequate strength, it was necessary to cold work these materials to varying degrees and such cold work adversely afiected the formability of the steel and limited the steel from the standpoint that fusion welding could not be employed since the welding procedure destroyed the effects of cold work thereby impairing the strength of the steel.

More recently a new family of steels which possesses the desired combination of both the A181 300 and 400 Series stainless steels, has been developed which has a controlled transformation precipitation hardening reaction designed to specifically overcome some of the limitations of the martensitic stainless steels and the austenitic stainless steels, as discussed hereinbefore. These steels have been known to the trade as the PH stainless steels and possess an excellent combination of fabricating and "ice engineering properties. The steels, however, possess a disadvantage in that their delicate metallurgical balance and complicated heat treatments reflect higher cost in processing, as well as fabrication and, as a result thereof, these steels uses are generally limited to aircraft and missile applications.

The steel of the present invention does not require the delicate metallurgical balance and is not a precipitation hardening stainless steel, yet possesses a martensitic microstructure with optimum formability together with high strength, adequate corrosion resistance, simplified heat treatment and excellent weldability, thereby combining advantageous properties of both the 300 and 400 Series stainless steels without the particular disadvantages encountered in the PH steels. In addition, the steel of the present invention possesses better modulus values than temper rolled austenitic stainless steels, which modulus values, in the steel of the present invention, are nondirectional thereby making the steel of the present invention highly attractive for structural applications.

An object of the present invention is to provide a high strength, martensitic, corrosion resisting steel.

Another object of this invention is to provide a high strength, martensitic, corrosion resisting steel characterized by having low hardness, good ductility and high strength which can be developed by a simplified heat treatment.

Another object of this invention is to provide a high strength, martensitic, corrosion resisting steel having an optimum combination of forming and welding character istics and which can be employed as a structural member in the transportation industry.

A more specific object of the present invention is to provide a martensitic, high strength, corrosion resisting steel containing carbon, silicon, manganese, chromium, nickel and titanium, in which the titanium to carbon ratio and the silicon content are limited to specific ranges.

Other objects of this invention will become apparent to one skilled in the art when read in conjunction with the following description.

In its broader aspects, the steel of the present invention has a composition which includes from traces to 0.1% carbon, from traces to 0.5% silicon, from traces to 1% manganese, from 10.5% to 14% chromium, from 3% to 6% nickel, titanium in an amount equal to from 8 to 18 times the carbon content, and the balance essentially iron with incidental impurities. Within the foregoing broad aspect of the steel of the present invention the alloying components contained therein perform functions as related hereinafter.

The carbon content of the present invention is limited to an amount between traces and 0.1%. It is preferred to maintain the carbon content as low as possible and preferably not above 0.05%, in order to obtain a substantially carbon-free martensite after cooling to room temperature from a low austenitizing heat treatment temperature. While carbon contents in excess of 0.05% and up to about 0.1% can be utilized, these higher carbon contents must be balanced with the titanium content'so as to insure an exceedingly low or substantially carbon-free martensitic structure which will have low hardness and good ductility. Thus, the use of carbon within the range between 0.05% and 0.10%, while permissible, is discouraged since the amount of titanium necessary to tie up the excess carbon results in a steel having an excess of intermet-allic compounds which are not useful and may in tween 3.5% and about s,ass,e 11

hibit the physical, chemical and mechanical properties of the steel.

The steel of the present invention contains silicon which must be limited to about 0.5% maximum. Silicon has been found to be a potent solution hardener which can have an embrittling effect and, as a result thereof, the silicon content must be maintained at no greater than 0.5 in order to obtain the proper degree of formability in the steel. While the silicon contents are preferably maintained low, it will be recognized that some silicon will always be present. Optimum results appear to be obtained where the silicon content is maintained from traces to about 0.30%. It is also preferred to maintain the manganese content on the low side in order to insure adequate rollability and good ductility to the material. In this respect, it has been found that up to about 1.0% manganese can be utilized; however, it is preferred to maintain the manganese content at less than 0.5%. Moreover, with low manganese and low silicon contents combined with a low carbon content, the cleanliness of the steel is greatly improved which aids in the corrosion resistance inherent .within the steel, but not fully developed unless the silicon,

manganese and carbon are maintained preferably as low as possible. I

The steel of this invention contemplates a chromium content within the range between about 10.5% and 14% and preferably within the range between 11% and 13%. At least 10.5% chromium is necessary in order to confer an adequate degree of corrosion resistance, whereas increasing the chromium content to beyond 14%, results in the formation of a duplex microstructure which will include delta ferrite, a component which will not transform to maitensite upon subsequent cooling to room temperature. Moreover, such a duplex microstructure which contains delta ferrite, adversely affects the mechanical properties, especially in the transverse direction, and re sults in increased difliculties during hot working operations. The optimum combination between corrosion resistance, strength, formability stability, appears to be obtained when the chromium content is present within the range between about 11% and about 13%.

As stated hereinbefore, it is desirable to produce a structure in the steel of the present invention which is substantially completely austenitic at a temperature of about 1400 R, which structure may be transformed to a substantially completely martensitic microstructure upon cooling to room temperature. For this reason, the steel of the present invention contains nickel in an amount between 3% and 6 and preferably within an amount be- At least 3% nickel is necessary in the steel of the present invention to form the austenite phase and to suppress the formation of delta ferrite, whereas nickel contents in excess of about 6% may stabilize the austenitic structure to a sufficient degree that austenite may be retained upon cooling to room temperature. Moreover, if the nickel is increased to beyond about 6%, some age hardening may be apparent Within the steel of the present invention, depending upon the relative level of the carbon and titanium present. Optimum results appear to be obtained when the nickel content is maintained within the range between about 3.5% and about 5.0%.

Titanium is also included Within the steel of the present invention and functions to balance the composition by uniting with the carbon and nitrogen contents so that 'when the steel is cooled to room temperautre a substantially carbonand nitrogen-free martensite is formed, which is characterized by low hardness and good ductility. For this reason, it is desirable to have the minimum titanium content present within the steel at about 8 times the carbon contained therein. While larger amounts of titanium can be utilized, for example a titanium content up to about 18 times the carbon content, higher titanium contents should be avoided because of the possibility of forming an age hardening component within the steel of the 'present'invention. This age hardening component depends upon the relative amounts of titanium and nickel present within the composition. Optimum results appear to be obtained where the titanium content is maintained within the range between about .20% and about .75 the titanium and carbon varying in relation to each other. While the low carbon and, consequently, lower titanium contents are preferred, a.- titanium to carbon ratio of at least about 8 should be present at all times.

The steel of the present invention may optionally contain columbium, molybdenum or vanadium for special purposes, it being noted that the columbium can be substituted for the titanium on the basis of about 2:1. Vanadium may also be employed to unite with the carbon thus removing it from solution within the martensite and thereby obtain relatively the same characteristics. Molybdenum may also be optionally added to the steel of the present invention, where added resistance to a corrosive environment in which a halogen ion is present, is desired. Molybdenum also improves the high temperature properties of the steel. The balance of the steel cornprises essentially iron with the usual impurities normally found in steel-making practice.

Reference is directed to Table I which sets forth in tabular form the general and optimum ranges of the composition of the steel of the present invention.

Element General Range Optimum Range Traces to 0.10 Traces to 0.05. Traces to 0.50 Traces to 0.30. Traces to 0.50.

8 X Bal The steel of the present invention may be made by any of the well-known steel-making practices, for example, the carbon electrode electric arc furnace, the details of which are well known in the art and need not be set forth in detail. The steel of the desired composition, as set forth hereinbefore in Table I, is cast into ingots which are hot rolled to any desired shape, for example, bloom, bar, billet, slab and fiat rolled products. After descaling, the steel may be cold rolled to finish gauge, for example, into the form of strip, wire, plate, sheet, tube, bar, rod or any other semi-finished mill product. It is to be noted that the steel of the present invention can be cold rolled in excess of 75% reduction in cross sectional area in fiat rolled product form Without any intermediate reannealing. The steel in the form of wire has been drawn to effect a reduction in the cross sectional area of up to 99% without any intermediate reannealing.

As cold worked, the steel may be heat treated in order to obtain adequate ductility for any of the various fabricating procedures, for example, drawing, bending and joining. It is preferred to heat treat the steel of the present invention after cold working to the finished mill product by heating to a temperature within the range between about 1400 F. and about 1600 F. After heating in this temperature range, the steel may be quenched in order to form a substantially completely martensitic microstructure which is characterized by high strength, low hardness, good formability, and with excellent welding characteristics.

Reference is directed to Table II which illustrates the typical mechanical properties of the steel of this invention, said steel having a composition which includes: 0.015% carbon, 0.13% manganese, 0.061% silicon, 11.0% chromium, 4.0% nickel, 27% titanium, and the balance essentially iron with incidental impurities. For comparative purposes typical properties of AISI Type 430 stainless steel, a ferritic stainless steel; AISI Type 302, an austenitic stainless steel, and AISI Type 410, a martensitic stainless steel, are included.

Table II TYPICAL MECHANICAL PROPERTIES Steel Condition .2% Y S UTS Percent (K p.s.i.) (K p.s.i.) El

This Inv- Hot rolled 131.1 138. 2 11.0 This Inv Annealed 105. 3 123. 8 8. 0

do. 50 75 26 o 30 93 65 hard 110 150 Annealed 32 60 1,800 E, 0Q,+1,000 F 117 131. 4 12 From the test results recorded in Table II which illustrate typical mechanical properties of sheet material, it is apparent that the steel of the present invention possesses high strength in the annealed condition as well as the hot rolled condition, which strength levels exceed those of any of the standard AISI type steels in the annealed condition. While both the .2% yield strength and ultimate tensile strength are higher and the elongations somewhat lower than the standard AISI steels, yet the elongation of this material is adequate and does not impair the formability characteristics of the steel. Data has also been included in Table II relating to AISI Type 302 in the annealed and the hardened conditions as well as AISI Type 410 in the annealed and the hardened conditions. From the mechanical strength standpoint, the steel of the present invention compares quit efavorably with both AISI Types 302 and 410 in the hardened conditions. However, since the steel of the present invention possesses said mechanical properties in the annealed condition, the formability, weldability and drawability are not adversely affected.

cross sectional area that the steel of this invention has a very low rate of work hardening thereby making the steel exceedingly attractive from a formability point of view.

In this same vein, the titanium to carbon ratio becomes very important especially where the transverse bending properties of the steel are considered. The transverse bending properties are those properties which are measured by the ratio of the bend radius to the thickness of the steel when said steel is deformed to produce a 135 bend without failure in the steel. It is considered that the bend ratio must be limited to not more than about 2 in order for the steel to have adequate transverse bendability which is a measure utilized in determining the formability characteristics of the steel.

Reference is directed to Table IV which contains the chemical composition of a series of steels having a varying titanium to carbon ratio, some of which are outside the scope of this invention, and which were utilized in order to evaluate the formability characteristics of the steel of the present invention.

As stated hereinbefore, the steel of the present invention can be cold worked to varying degrees Without the necessity of an intermediate annealing heat treatment. This results from the fact that the low carbon martensite has a very low rate of work hardening which is directly The steels having the composition set forth in Table IV were rolled to varying sheet thicknesses and were subjected to the standard hardness, tensile and bend tests. These tests are set forth in the following Table V. In each instance, the steel was given an annealing heat treatcontrary to the AISI 300 Series stainless steels. Referment at a temperature of 1400 F. following which the ence may be had to Table III which shows the effect of steel was air cooled to room temperature. cold work on a heat of steel having a composition of Table V 0.029% carbon, .13% manganese, .06% silicon, 11.04% chromium, 3.27% nickel, 23% titanium, and the balance EFFECT OF Q essentially iron with normal impurities.

Table II 1 Heat No. Ti/C Hardness UTS R/T Bend Re (K p.s.i.) Ratio EFFECT OF GOLD WORK 3. 64 39. 5 165. 7 3.1 Condition Hardness .2% Y8 UTS E1 Percent 5. 30 34. 0 159. 3

Re (K p.s.i.) (K p.s.i.) 6.35 35. 0 161.3 3.1 8.85 27. 5 135. 5 1. 6 9. 25 26. 0 125. 0 1. 0 28.0 111.8 130.9 6.5 GM-96 16.65 24.0 122.8 1.0 28.0 125.6 132. 6 6. 0 31. 0 142. 2 147. 9 4. 0 34.0 154.7 165.1 2.5 I The test results set forth in Table V clearly illustrate increase is noted in the ultimate tensile strength and a decrease in the ductility as measured by the percentage elongation, it is apparent from the relatively small increase in tensile properties after cold reductions of 75 of the that where the titanium to carbon ratio is less than about 8, increased hardness is noted. This is also reflected in the ultimate tensile strength and when the bend ratio is considered, it is clear that the bend ratio is greater than about 2. On the other hand, where the titanium to carbon ratio is maintained within the range set forth hereinbefore in Table I, it is clear that not only does the hardness drop, but also the bend ratio decreases to less than about 2. While a decrease in the ultimate tensile strength is noted where the Ti/ C ratio exceeds 8, the relative level is sufficiently high so that the steel will exhibit good mechanical properties which are superior to those exhibited by either the standard martensitic, ferritic or austenitic stainless steels of the AISI series in the annealed condition.

As set forth hereinbefore, the steel of the present invention possesses excellent welding characteristics. The steel having a composition of .015 carbon, .0l9% silicon, .14% manganese, 10.92% chromium, 4.05% nickel, .25 titanium, and the balance essentially iron with impurities, was cold rolled to strip material having a thickness of 0.062", annealed and was welded by the Tungsten Inert Gas Process, there being no filler metal added during such welding operation. The steel was welded without benefit of a prewelding heat treatment and the properties set forth hereinafter in Table VI were the properties in the condition stated without benefit of a postwelding heat treatment.

8 While it would appear that the welded metal has a higher elongation than that of the parent metal, the data set forth in Table II is for sheet material, Whereas the data set forth in Table VII relates to bar stock. When the notched properties were evaluated, it was clear that the steel in the welded condition was notch ductile, exhibiting a ratio of notch tensile strength to ultimate tensile-strength greater than 1. These properties are obtained in both the as-welded and in the stress relieved condition, it being noted that there was no preheating or postheating treatment applied to the steel during the welding operation. Substantially similar results were obtained with electrical resistance spot welds. Since the as-deposited weld metal properties are outstanding, it is clear that the steel of the present invention can be utilized in joining any high strength stainless steel without the necessity of preheating or postheating treatments unless the base metal re- Table VI WELDING PROPERTIES Condition Type Test .2% YS UTS E1 Percent Site of (K p.s.i.) (K p.s.i.) Fracture Annealed at 1,600 F Unwelded 110.0 133.0 3. 5 Aged 5 min. at 1,000 F Unwelded 118.0 133.0 9. 5 As welded Transverse to weld 111. 1 122. 3 5. 0 Base metal. Welded plus 5 min. at 1,000 Transverse to Weld- 114.0 121. 0 6. 5 Basemetal.

From the test results recorded in Table VI, it is clear that very little change is noted in the mechanical properties exhibited by the parent metal in the unwelded condition and those of the welded metal when the steel was tested with the weldment transverse to the direction of testing. This holds true both for the yield strength as well as the ultimate tensile strength and, while some difquires the same. This property is unique among the known martensitic stainless steels.

The steel of the present invention exhibits corrosion resistance which compares quite favorably with both AISI Type 430 and Type 410. In particular, the steel having a composition within the range set forth hereinbefore in Table I, was subjected to the following listed acid solutions ference is noted in the elongation as measured, the locafor the time periods indicated and the weight loss calcution of the fracture was in the base metal which clearly lated in inches penetration per month was determined both indicates the outstanding welding characteristics of this for the steel of the present invention, as well as standard steel. An aging at 1000 F. for 5 minutes shows a slight AISI Types 410 and 430.

Table VIII CORROSION RESISTANCE Weight Loss (1PM) Solutions Time This invention 410 430 boiling Nitric Acid Averagg 4-48 hour 0125 G153 0007 110 60% boiling Acetic Acid 24 1i rs 0347 1598 025 20% boiling Phosphoric Acid 24 hrs 0009 0481 015 improvement in the yield strength both in the unwelded It is clear from the test results set forth hereinbefore in and welded conditions. This behavior demonstrates that Table VIII that the steel of the present invention has a no postwelding treatment is necessary. corrosion resistance with respect to nitric acid which is Steel having the same composition was rolled to plate substantially intermediate Type 410 and Type 430. This having a thickness of /2 and thereafter the plate sois probably due to the fact that the steel of the present produced was subjected to multiple pass welds using the invention has less carbon than Type 410 and less chromitungsten inert gas and welding wire of the same comum than Type 430. When the same steels are tested in position as the parent metal. After multiple pass weldacetic acid substantially similar results are obtained. On ing, both smooth bar and notch bar samples were made the other hand, when the stated phosphoric acid atmosfrom all weld metal and were tested, the results of which phere is used, it is clear that the steel of the present invenare set forth in Table VII:

Table VII Condition Type Test 2% Ys UTS E1 RA NTS N'rs/ (K p.s.i.) (K p.s.i.) (percent) (percent) (K p.s.i.) UTS As welded A11 wtelld 113.9 133.2 14.0 67 189.2 1.4 As welded-F4 hrs. Aii iiia 114.9 116.9 21.4 57 190.3 1.6

at 900 F. metal.

As clearly illustrated, the as welded and the welded and heat treated conditions of the metal exhibit properties quite similar to those of the parent metal, both in tion possesses a corrosion resistance which is more desirable than either that of AISI Type 410 or Type 430.

In addition to the foregoing test results, samples of the the yield strength and in the ultimate tensile strength. steel of the present invention as well as Types 410 and 430 were subjected to a 5% neutral salt spray atmosphere with the results that the steel of the present invention shows only a few rust spots after 200 hours exposure. On the other hand, Type 410 shows a rusting in 20 hours and AISI Type 430 appears about the same as the steel of the present invention after about 72 hours exposure to said 5% neutral salt spray. Based on the foregoing, the steel of the present invention possesses corrosion resistance properties Which are highly desirable where the steel is employed in structural applications.

The steel of the present invention has been utilized in flat rolled product form, for example, sheets and plates, and has been formed specifically into corner posts for automotive trailers, and in all operations, that is, melting through semi-finished mill products and into the final fabricated form, standard equipment presently in use in these industries today has been utilized Without difiiculty.

We claim:

1. A martensitic stainless steel consisting essentially of from traces to 0.10% carbon, traces to 0.5 silicon, traces to 1.0% manganese, from 10.5 to 14% chromium, from 3% to 6% nickel, from 8 to 18 times the carbon content of titanium, and the balance essentially iron with incidental impurities.

2. A martensitic stainless steel consisting essentially of about 0.015% carbon, about 0.13% manganese, about 0.061% silicon, about 11.0% chromium, about 4.0%

nickel, about 0.27% titanium, and the balance essentially iron with incidental impurities.

3. A martensitic stainless steel consisting essentially of from traces to 0.10% carbon, t-races to 0.5% silicon, traces to 1.0% manganese, from 10.5% to 14% chromium, from 3% to 6% nickel, from 0.2% to 0.75% titanium, the ratio of titanium to carbon being Within the range between about 8 to about 18, and the balance essentially iron with incidental impurities.

4. A composition suitable for welding dissimilar metals, characterized by a low rate of Work hardening, good f-ormability and corrosion resistance, said composition consisting essentially of from traces to 0.1% carbon, from traces to 1% manganese, from traces to 0.5% silicon, from 10.5% to 14% chromium, from 3% to 6% nickel, from 8 to 18 times the carbon content of titanium, and the balance essentially iron with incidental impurities.

References Cited by the Examiner UNITED STATES PATENTS 5/1925 Kuehn 75-128 9/1961 Lula 148-142 

1. A MARTENSSITIC STANILESS STEEL CONSISTING ESSENTIALLY OF FROM TRACES TO 0.10% CARBON, TRACES TO 0.5% SILICON, TRACES TO 1.0% MANGANESE, FROM 10.5% TO 14% CHROMIUM, FROM 3% TO 6% NICKEL, FROM 8 TO 18 TIMES THE CARBON CONTENT OF TITANIUM, AND THE BALANCE ESSENTIALLY IRON WITH INCIDENTAL IMPURITIES. 